For safety and service reasons nitrogen is used in the aircraft and car racing industries for filling tyres as well as a compression/expansion gas for hydraulic systems.
Will we get any benefit from pumping our tyres with a different type of gas rather than air?
Absolutely not!. There will be no performance improvements and no negligible maintenance difference at all.
Indeed, it’s only practical to inflate our tyres with just few gases apart from common air. Let’s see why.
Hydrogen and helium are the two smallest and lightest atoms. On the other hand the smallest diatomic molecule that can be found in nature is hydrogen gas (H2 ). Other diatomic molecules are O2 (oxygen), NO (nitrogen oxide) and CO (carbon monoxide). Air contains 78% of N2 , 21% of O2, 1% of argon and little amount of other gases.
In this scenario hydrogen and helium stand out as potential candidates due to their light nature, specially hydrogen.
The reality is that in the case of hydrogen it will leak out from tyres much faster than air. Not because its reduced molecular size. Hydrogen permeability with butyl compounds is all about. Hydrogen diffuses through the inner tube wall faster than other more rubber compatable gases. It’s a fact that helium or hydrogen dissolution inside the rubber matrix will end up in continuous top-ups, if not during our rides reducing a lot the interval service.
For example, CO2 contained in re-fill bottles, despite it’s higher molecular weight and size, has proved conclusively to leak out faster than regular air due to a higher solubility in butyl rubber which allows it to diffuse through the inner tube wall faster than smaller gas molecules.
Hydrogen and helium are then discarded within the options of inflating tyres for obviuos reasons. We will only refer to them again to talk about weight savings.
But will the nitrogen alternative be suitable?
Yes it, is. But it does not provide any advantage inside bicyle tyres.
Nitrogen gas is present in aeroplanes and racing tyres. It is used in such situations because it’s drier than air. Even though air with free moisture contain is enough in certain applications.
Secondly, nitrogen is less chemically social. Although nitrogen is not formally an inert gas, it is however considered as such as it does not readily react with many elements whereas oxygen contained in air reacts violently.
Let’s evaluate several tyre parameters in extreme conditions to determinate if there is any gain of utilising nitrogen in bike tyres.
Pressure changes with temperature.
The volume of regular air will change with heat and thus pressure. Air contains water in vapor state (in both liquid and gas forms). While the temperature rises, moisture turns to gas vapor which increases the volume tremendously at certain point. And this variation is on top of the normal rate of dilatation of gases. An undesirable phenomenon where pressure stability is required.
For car racing the main problem is to keep moisture out of the inside of the tyre so that pressure is more consistent. Most of the wheel performance and safety issues in motorsports are solved by using dried-air and therefore no moisture is present. The Formula One competition has been reported to use air in compressors that only have water extractors.
Where tyre friction is critical nitrogen is preferred. Nitrogen is popular in Nascar racing for avoiding tyre explosions due to moisture overboiling and oxygen reacting with high flammable rubber compounds. It is appealing as well where large volume of air is inside the tyre, and that means a lot of water that affects the pressure with temperature above the normal rate of gas expansion. This is the case of the aircraft industry.
Bike tyres don´t heat up as much as car tyres do. For a bicycle tyre, the volume of air is very small and usually inflated to high pressure. Any expanding accounted to water molecules will not be noticeable, and unlike high temperature apps, less than accounted on normal air expanding with temperature, which is already very low.
Even on pristine downhill races or road tubular tyres competitions on hot days the pressure discrepancy is negligigle.
This is a major problem in aeroplanes, rare in motorsports and no applicable to bicycles.
When taking off and landing a tyre can get high temperatures and pressure changes. Water vapor content that can boild up increasing the tyre temperature and also elements like oxygen that can react with rubber vapors or wheels metals and become explosively expansive in the presence of enough oxygen. Consider that rubber compounds are highly flammable. In that case nitrogen will be less likely to react avoiding ignition. Nitrogen actually helps to extinguish any fire.
Lifespan of the tyre.
Air if not dried contains moisture that adds to the corrosions concerns of oxygen. Nitrogen will prevent internal rust in parts because there is no oxygen to react with other elements and because we get it dry out of the compressors.
The use of an inert gas could be important in accumulators, hydraulic pre-charge systems, surge suppressors and fire extinguiser bottles but no concern on rubber rusting up.
In any case, performance tyres on cars and bicycles are frecuently replaced though. Note that corrosion is a long-term process.
Here a question arises: Is nitrogen less porous than other gases and keep the right pressure longer than air?
Air, in fact, is made of 78% nitrogen itself as we said at the beginning of this article. So nobody expects big differences for just a 22% variation caused mainly by oxygen molecules.
But the reality is that nitrogen, being a smaller and lighter molecule than oxygen will last longer inside a tyre unlike the wrong premise of many car repair shops claiming nitrogen bigger size to be the reason to retain optimal pressure longer in compare with air.
How is possible that nitrogen diffuses slower than oxygen contradicting Graham’s law?
Well, nitrogen, despite its minimally smaller size versus oxygen, is more compatable with rubber compounds and less likely to trespass the atomic structure of inner tubes/tyres. Its worse solubility makes the gas to stay longer but do not forget air is still mainly nitrogen!
Nitrogen filled tyres for general consumers if not a scam, will elongate the gas refills periods in a 10%-20% at the most. I have not seen any literature proving its effectiveness in standard environments. Probably at high temperatures moisture can play a key role in the volume of air and pressure, but it sounds like science fiction that replacing oxygen molecules for fatty nitrogen ones is going to reduce the rate at which compressed gas diffuses through porous walls leading to more uniform tyre wear and better fuel consumption.
If nitrogen molecules was way less permeable than oxygen you would get this paradox:
Since oxygen leaks out faster leaving nitrogen molecules inside my tyre, in successive air refills I would increase nitrogen percentage each time. After ten or twenty top-ups my tyres would be mainly keeping nitrogen inside 🙂
Did I say not to bother on bicycle wheels where you can come across with sealant filled tyres to ensure self-healing (and losing air in that process) or very porous latex inner tubes that do not keep the optimal pressure for more than a ride?
On a bicycle the added weight of air per wheel is around 18 grams. Can we cut it down?
O2 weights 32 grams/mole and N2 weighs 28 grams/mole. Since air is about 78% N2, the average weight is taken at 28.9 grams/mole, which means you get a ridiculous 3% weight savings in compare to air. Less than a gram per wheel.
Theoretically, hydrogen is 1440% lighter than air (and so is helium – 724%) so we could end up in a almost 18 grams per wheel less. Unfortunately both gases are useless to conserve pressure as explained before.
Why normal air is so good for general purposes?
Because it is made of 78% of nitrogen and air humidity is a minor issue that only counts in high temperature applications.